Improve API reference doc structure

[dstansby]

This improves the structure of the API reference by:

• Generating the pages under api/, instead of redirecting from an index page under api/ to _autoapi/
• Adding a redirect from the v2 API page to the new page
• Before you had to click on "API reference", then "zarr" on the following (mostly empty) page to get to the ref. Now "API reference" links directly to the API reference page.

[jhamman]

Great improvement. Happy to approve once this is ready for review.